ORIGIN OF SOIL 44 
scarce or wanting in barren soil. Though its chemical value is 
too complex to be stated or even known, its origin is easy to 
understand. 
Humus is the remains of life of previous generations. When 
plants die, their roots, together with their leaves, branches, and 
fruits, inevitably become incorporated into the soil. Animals, too, 
leave upon the ground a quantity of excrement and other dis- 
charges; and plants likewise probably discharge excretions into the 
soil. When animals die their bodies also may become mixed with 
the earth. Thus, practically all kinds of organic matter from 
animals and planis are being mixed continually with mineral 
ingredients in the surface layers of the soil. The microdérganisms 
in the soil feed upon these dead materials, causing an extensive 
series of decompositions and recombinations. To this mass of 
complex organic bodies undergoing decomposition in the soil has 
been given the name humus. It will be evident from this explana- 
tion of its origin that humus cannot have a definite composition, 
and that it will hardly be alike in any two soils. It will be com- 
posed of different materials to start with, and there will be a 
variety of different stages of decomposition. We cannot hope to 
find any definite composition of humus, but we can study the 
kinds of decomposition and recombinations that are going on In it 
and that result in making it a suitable food for plants. In this 
study we must ever keep in mind the fact that dead bodies of 
animals and plants are not in condition to serve another generation 
of plants as food. We cannot feed plants upon eggs, or urine, or 
starches, or sugars. Though containing carbon and nitrogen in 
abundance, these elements are locked up in them out of the reach 
of the green plants, and before they can be utilized again they must 
be freed from their combinations and brought into simpler forms. 
This is accomplished by the microdrganisms in the soil. Our 
study of these changes may best be centered around the two 
chemical elements, carbon and nitrogen. 
